This invention relates to a manifold apparatus for a plurality of liquid filtration segments wherein filtration is effected by tangential flow of the liquid across a filter surface. More particularly, this invention relates to a manifold which permits stacking of a plurality of filtration modules wherein liquid is introduced and removed from the filtration module under conditions to provide improved separation of filtrate species from retentate species.
Prior to the present invention, liquids, primarily aqueous liquids, have been filtered by tangential flow modules wherein a plurality of filters are stacked between two manifolds, one at the bottom of the stack and one at the top of the stack. The filtration module stack comprises a collection of one or more filtration modules and associated intermodule gaskets. The filters are separated by appropriate screens and liquid-impervious layers to permit liquid flow, as well as filtrate and retentate flow into and from the stack. In addition, flow holes are provided through the layers within the stack. The holes are sealed appropriately to prevent admixture of filtrate with incoming liquid feed or retentate. The manifolds in each segment are designed so that filtrate is removed from both the top and bottom manifolds while incoming liquid is introduced into one manifold and retentate is removed from the second manifold. This manifold arrangement necessitates fittings to both manifolds in order to supply and remove liquid to and from the stack. This is undesirable since additional manipulative steps for removing and connecting the fittings are required when it is desirable to replace the filtration elements forming the stack.
A common problem encountered with the use of tangential flow filtration modules is the formation and growth of a retentate boundary layer on the filter surface adjacent the fluid inlet or inlets to the retentate surface of the filter. The formation of this layer reduces the efficiency of the filter and mitigates against fine separation of constituents in the fluid feed. This, in turn, results in undesirable retention of desired portions of the filtrate in the retentate. It is believed that a prime cause of the formation of the boundary layer is the relatively high pressure drop along the retentate side of the filter, resulting in high feed inlet pressure.
It has been proposed in U.S. Pat. No. 4,849,102 to provide a manifold for a plurality of filtration modules. However, this manifold arrangement is limited to only one manifold while avoiding the need for conduits external the manifold. That is, when more than one manifold is utilized, external connecting conduits are required.
It has been proposed in U.S. Pat. No. 5,147,542 to provide a manifold apparatus for tangential flow filtration modules which eliminates the need for conduits outside the manifold apparatus and the filtration modules to attain desired flow of fluid feed, filtrate and retentate. It has also been proposed in U.S. Pat. No. 5,176,828 to provide a manifold apparatus for tangential flow modules at a central section within a stack of filtration modules in order to increase throughput rates of a fluid feed as compared to such an apparatus wherein a fluid is fed from an end of the stack of filtration modules.
Accordingly, it would be desirable to provide a manifold for a plurality of tangential flow filtration modules which accommodates any number of a plurality of filtration modules and manifolds and which provides a means to control pressure drop across a filter layer within the filtration module. In addition, it would be desirable to provide such a manifold which does not require additional conduits outside the manifold apparatus.